Dave,it doesn't mention a paint mark in the rules and that is why it's allowed. Many of the rules are about what is specifically not allowed, so a paint mark is allowed. I don't know the exact sight the OP is talking about but I would guess it's an addition to a front sight and that is what typically DQ's it. A small paint mark can be removed if a ref had a problem but i've never seen or heard that. Lots of guys have made a small mark in the face of the blade, even G.I. national match slides came with front sights that were serrated. I'm not in the business of splitting hairs but I cannot count the number of front sight blades, and front sights on EIC AR15's that have a small paint mark on them. You don't have to if you don't want.